220 COMPARATIVE ZOOLOGY. 



their young, i. e., they are Mammals. All Rumimuit 

 hoofed beasts have horns and cloven -feet. If the hoofs 

 are even, the horns are even, as in the Ox ; if odd, as in 

 the Rliinoceros, the horns are odd, i. e., single, or two 

 placed one behind the other. Recent creatures with feath- 

 ers always have beaks. Pigeons with short beaks have 

 small feet; and tliose witli long beaks, large feet. The 

 long limbs of the Hound are associated with a long head. 

 A white spot in the forehead of a Horse generally goes 

 with white feet. Hairless Dogs are deficient in teeth. 

 Long wings usually accompany long tail-feathers. White 

 Cats with blue eyes are usually deaf. A Sheep with nu- 

 merous horns is likely to have long, coarse wool. Homol- 

 ogous parts tend to vary in the same manner; if one is 

 diseased,' another is more likely to sympathize with it than 

 one not homologous. This association of parts is called 

 correlation of growth. 



6. Individuality. 

 It seems at first sight very easy to define an individual 

 animal. A single Fish, or Cow, or Snail, or Lobster is 

 plainly an individual; and the half of one such animal is 

 plainly not one. But when we consider animals in colo- 

 nies, like Corals, it is not so easy to say whether the indi- 

 vidual is the colony or the single Polj'p. Is the tree the 

 individual, or the bud i If we say the former — the colony 

 — what shall we say to the free buds of a Hydroid colony, 

 living independent lives, and scattered over square miles 

 of ocean ? Are they parts of one individual ? If we 

 choose the latter as our standard, we are in equal difficul- 

 ty; for we must then call an individual the bud of the 

 Portuguese man-of-war, reduced to a mere bladder or 

 feeler, and incapable of leading an independent life. We 

 thus find it necessary to distinguish at least two kinds 

 of individuals — physiological individuals, applying that 



